The firefighters’ union is suing the FDNY, claiming officials are using 9/11 as an excuse to reduce manpower in more than 50 engine companies.

The lawsuit filed by the Uniformed Firefighters Association seeks to stop the city’s plan to cut 53 city engine companies from five members to four, a move that could save the city $12 million.

The union argues the city and the FDNY are unfairly using a loophole in their contract with the city that allows them to cut staff when the number of firefighters on sick leave hits 7.5 percent.

The union says many firefighters are still suffering from aftereffects of 9/11 – and the numbers are high because the FDNY is down about 600 firefighters due to Sept. 11 losses, retirements and a hiring freeze.

FDNY spokesman Frank Gribbon said the union was using 9/11 as a pretext, adding it had signed an agreement in 1996 that allowed for the cuts.

Fewer than 10 firefighters remain on medical leave as a result of the attacks, Gribbon said.

“We had an agreement,” he added. “Now they’re saying it’s a loophole because of 9/11? That’s just not right.”